Witwatersrand

(Map) WITWATERSRANDWITWATERSRAND (EN: Witwatersrand; RU: Витватерсранд) is the ore region within the Republic of South Africa, which is including the complex deposit of gold and uranium of the same name, which is unique in terms of the reserves. This region is situated within the Transvaal and Orange provinces. The Witwatersrand deposit has been discovered during the 1886, the extraction of gold has been started at the same time, and the extraction of uranium has been started since the 1952. The centres of the developments within the Witwatersrand region are the Johannesburg, Klerksdorp, Odendaalsrus, and Welkom cities. The deposit belongs to the type of the ancient metamorphosed conglomerates. The mineralization is mostly confined to the Witwatersrand system of the Lower Proterozoic (see the Map). The ore-bearing stratum forms the synclinorial structure, which is complicated with the projections of the Archean basement. The ore bodies (the so-called reefs) are represented as the packs of the ore-bearing conglomerates with the interlayers of the quartzite without any ore. The thickness of the individual industrial layers of the conglomerates is up to 4.5 metres. During the 1976, gold (50%) was being extracted from the layers with the thickness of less than 0.3 metres. The region of the distribution of the gold ore bodies occupies the area of approximately 350x200 kilometres. The development has been started at the north, near the Johannesburg city. On the new areas within the Orange province, the ore-bearing stratum is overlain with the horizontally embedded layers of the Karoo formation (Carboniferous-Permian) with the thickness of more than 300 metres.

The ore mineralization is located within the oligomictic conglomerates. The pebbles (70%) are represented as the veinous quartz, and in the subordinate manner as the quartzite. The mica-quartz cement contains 2-16% of pyrite, and yet approximately 50 other minerals. Gold is located within the pyrite, and also is isolated through the microcracks, which are dissecting the quartz pebbles and cement. The average size of the grains of gold is 5-100 micrometres, the millesimal fineness is 906-935. The uranium minerals are the uraninite, pitchblende, brannerite, and thucholite. There may be encountered the diamonds, and the minerals of the platinum group.

The genesis of the Witwatersrand deposit is subject to discussion. There are most universally accepted the ideas about the initial accumulation of gold and uranium within the alluvial coastal-marine placers, with the subsequent significant regrouping of these placers. There were being developed the thesises about the substantial participation of the hydrothermal processes in the genesis of the deposit.

Prior to the 1980, there has been extracted from the Witwatersrand deposit 36 thousand tonnes of gold, which amounts to approximately 1/3 of the extraction of gold within the industrially developed capitalistic and developing countries. The extraction has reached the maximum during the 1970 (1000.4 tonnes), after which time this extraction has started to decrease gradually (703.3 tonnes during the 1979). The industrial reserves of gold are evaluated from 15.5 to 18.7 thousand tonnes. The average content of gold during the start of the operation was 17-20 grams per tonne, and during the 1978, the content of gold was 8.85 grams per tonne, with the strong fluctuations throughout the individual ore mines (from 5.7 to 28.2 grams per tonne in situ, and from 2.7 to 24.8 grams per tonne within the ore, which was being extracted). The content of the U3O8 compound is 0.02-0.05%. The extraction of uranium is from 3.2 (1976) to 5.5 (1979) thousand tonnes per year. The reserves of the uranium oxide are evaluated at 150-170 thousand tonnes. Along the way, there are extracted the pyrite, minerals of the osmiridium group, other platinum metals, and silver. During the 1978, approximately 98% of the total extraction of gold was accounted for 33 underground mines, 12 of which yielded 65%. The depth of the development is 1700 metres on average, the maximal depth is 3600-3800 metres. There are performed the activities for the intensification of the mining works: the selective extraction of the thin layers of the conglomerates, without the blasting works, with the usage of the pulsed hydraulic rippers, the sawing of the thin ore interlayers, and so on. The schematic diagram for the extraction of gold and uranium within the Witwatersrand prescribes the cyanidation, with the subsequent leaching of the uranium from the tailings using the sulfuric acid. They obtain this sulfuric acid from the same ore, during the roasting of the flotation concentrate of pyrite from the tailings of the acidic leaching. The total extraction of gold is approximately 90%.